5 Steps to Implementing GD&T in Your Part Drawings

Effectively Communicate Your Product's Functional Design Requirements with GD&T

As problems arise for your customers, you design products to solve those problems. Your engineering, development, and manufacturing teams rely on your designs to develop and produce your product.


Poorly defined specifications will have ripple effects that can last years. You do not want your drawings to fall into that trap. When you share drawings that include ambiguous, unfocused design intent, you confuse your audience and you end up with over-constrained tolerances that threaten the long-term quality of your program. We previously shared the three common mistakes in engineering drawings. You can view the webinar here or the slides from that webinar here

You want your part drawings to communicate your design intent in a clear, concise way. Geometric Dimensioning and Tolerancing tools offer a clear language that translates functional requirements into your design intent. These five steps will help you implement GD&T in your engineering drawings so you can improve the long-term quality of your product.

You can download a GD&T checklist here to bring consistency and specificity to your drawings.

1. Identify Your Functional Features

You start implementing GD&T by defining the functional features critical to device performance. A typical part can have dozens, or even hundreds, of dimensional callouts. However, only a handful of those dimensions will affect the part’s intended function. The interfacing assembly features on the bottom and top half of a molded cassette, such as a mating boss and hole, impact the part’s functionality.


2. Choose Your Controls

The published ASME-Y 14.5 guidelines provide 14 unique symbols to control form, orientation, size, or location. You want to select the control that most accurately captures the functional intent. Be careful not to select a control that unnecessarily overcomplicates the validation of the functionality.


3. Define Your Tolerances

You want to define an allowable amount of deviation from nominal. You should understand the upper and lower specification limits of your product. During the initial phase of development, you want to find out how good your part needs to be in order to work. Most product developers stop there and either use general tolerances or overconstrained tolerances. This sacrifices quality over the long term. Rarely do product developers continue to the next phase of development and discover how bad the part can be and still work. This informs your tolerances.


4. Define Your Datum References

You want to identify which external feature to reference. You can refer to this as your Datum feature. Some GD&T controls, such as form control, do not allow the use of a Datum in an evaluation and would have a completely defined callout with just the control characteristic and tolerance. Other control characteristics, such as orientation and location, will likely require a Datum to satisfy its evaluation.


5. Designate Your Datum Alignments

One or more Datum references noted at the end of a feature control frame signifies a critical feature to evaluate the selected control characteristic. A fully constrained datum reference frame will have a total of three Datum features. These Datum features create a theoretical intersection. This intersection establishes both a point of origin and an axis of alignment. In best practice, a fully constrained Datum alignment will restrict all six degrees of freedom about a part.

Datum features inherently define a common setup method that will promote repeatable inspection results, reproducible quality of manufacturing and will allow simulation of part assemblies without physically putting a device together. By using these five steps to implement GD&T in your drawings, you will achieve clarity in your designs, which will improve the long-term quality of your application.

I have helped many of my clients build their skills in GD&T application. If you need help validating your requirements and conveying true functional meaning in your part drawings, call me at (631) 285-2424.

Download GD&T Checklist

The GD&T Checklist includes 5 Steps to help you implement GD&T and a checklist that can bring consistency and specificity to your drawings.